1. Field of the Invention
The present invention relates to hydraulic systems that independently control the operation of a plurality of hydraulic actuators on a vehicle that is powered by an engine, and more particularly to such hydraulic systems that include a mechanism which prevents the engine from stalling when the hydraulic system suddenly demands increased power from the engine.
2. Description of the Related Art
Numerous types of machines have components that are moved by a hydraulic system. For example, a various vehicles used in construction and agriculture have an internal combustion engine which drives a pump to provide pressurized fluid for powering different hydraulic functions, such as lifting the objects and working the ground. This vehicle has body to which a boom is pivotally attached and able to be raised and lowered by a first hydraulic cylinder. A work member, such as a bucket or load carrying platform, pivots about the remote end of the boom in response to a second hydraulic cylinder. Pressurized fluid from the pump is applied to the first and second\ cylinders by the operator manipulating separate valves which control the rate and direction of fluid flow so that the cylinders operate in a manner that produces the desired motion of the boom or work member.
When the vehicle is stationary, the engine often operates at a relatively slow idle speed. In this situation, when the operator desires to raise the work member, the engine may not be able to supply sufficient horsepower to satisfy the demands of the hydraulic actuator. For example, the load member strikes mechanical stops at the extreme ends of the pivot motion. Keeping the associated control valve open thereafter causes pressure in the hydraulic system to rise dramatically. At idle speed the engine may be incapable of generating and sustaining the required torque necessary for the hydraulic pump to supply fluid at that increased pressure. In that case, the engine speed decreases rapidly until the engine stalls. The engine usually stalls before the pressure reaches a level at which a conventional pressure relief valve opened.
Heretofore engine stalling was prevented by placing a bypass path between the pump output and the return conduit leading to the system fluid reservoir. This path had an orifice through which fluid continuously flowed from the pump to the tank. This restricted flow path effectively reduced the opening pressure characteristic of the conventional pressure relief valve, thereby limiting the amount of pressure that the control valves for the hydraulic functions can provide when the pump output is low. However, this approach has the disadvantage of creating a continuous flow loss to the tank which in some applications was unacceptable because fluid is routed away from the active hydraulic actuator. For example in a lift truck, the flow through the bypass path significantly reduces the speed at which the load carrying member can be raised.
Therefore, a need exists form an improved mechanism to prevent the engine from stalling when the hydraulic system demands a level of power that can not be satisfied by the engine.